Apoptosis or programmed cell death is a cellular suicide process in which damaged or harmful cells are eliminated from multicellular organisms. Cells undergoing apoptosis have distinct morphological changes including cell shrinkage, membrane blebbing, chromatin condensation, apoptotic body formation and fragmentation. This cell suicide program is evolutionarily conserved across animal and plant species. Apoptosis plays an important role in the development and homeostasis of metazoans and is also critical in insect embryonic development and metamorphosis. Furthermore, apoptosis acts as a host defense mechanism. For example, virally infected cells are eliminated by apoptosis to limit the propagation of viruses. Apoptosis mechanisms are involved in plant reactions to biotic and abiotic insults. Dysregulation of apoptosis has been associated with a variety of human diseases including cancer, neurodegenerative disorders and autoimmune diseases. Accordingly, identification of novel mechanisms to manipulate apoptosis provides new means to study and manipulate this process.
The first “inhibitor of apoptosis protein” (IAPs) was identified in a baculovirus. The baculovirus IAPs, CpIAP and OpIAP, are able to block apoptosis induced by p35-deficient baculovirus AcMNPV in insect Sf-21 cells. Cellular IAP homologues have been found in various animal species including worms, insects and humans. IAP proteins have a distinctive primary structure. They contain one to three copies of “baculoviral IAP repeat” (BIR) domain and most IAPs also contain a RING domain near their C-termini. The BIR domain contains a highly conserved arrangement of Cys/His residues forming a stable fold that chelates zinc. The BIR region was also found to interact with regulators of IAPs including Grim, Reaper and Hid from Drosophila and may also mediate homo-oligomerization. The RING finger is a common zinc-binding motif that also exists in other cellular proteins. Recent studies show that several IAPs, including XIAP (a NF-kappaB-dependent member of the IAP gene family, see, e.g., Deveraux (1999) EMBO J. 18:5242-5251), cIAP1, CIAP2, DIAP1, SfIAP and CpIAP are inhibitors of caspases, a family of intracellular proteases responsible for the execution of the apoptosis program. These IAPs can directly bind and inhibit some members of caspase family including caspases-3, -7 and -9. Structure-function studies have demonstrated that inhibition of caspases-3 and -7 requires only a single BIR domain while RING domain may perform other functions including recruitment of ubiquitin conjugating enzymes (UBCs). When expressed without the associated BIRs, the RING region of SfIAP was found to enhance the proapoptotic activity of mammalian caspase-9 suggesting this domain operates as a trans-dominant inhibitor of endogenous proteins involved in apoptosis suppression. Ectopic expression of lepidopteran SfIAP and baculoviral CpIAP blocks apoptosis in mammalian cells, suggesting conservation of the apoptosis program among various species and a shared mechanism used by the IAP family.
Bombyx mori (silkworm) has been domesticated for silk-production for thousands of years. Used together with baculoviruses, it has also been developed as an organism for large-scale production of foreign proteins in the biotechnology industry. Despite its extensive use in sericulture and biotechnology, to date no apoptosis-regulating genes of silkworm have been identified.